Green’s Dictionary of Slang

busk n.

In phrases

make free with both ends of the busk (v.)

of a man, to caress a woman intimately.

c.1800		‘Bonny Lass o’ Liviston’ in Burns Merry Muses of Caledonia (1965) 158: Wi’ baith ends o’ the busk, I made me free.
1890–1904		Farmer & Henley Sl. and Its Analogues.

In exclamations

both ends of the busk! [SE busk, a corset, spec. its stiffening/supporting whalebone or other agent; the top would support the breasts, the bottom be near the vagina, the parts of the body that are being celebrated]

a toast before drinking.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Busk. A piece of Whalebone or Ivory, both formerly worn by Women to stiffen the fore of their stays: hence the toast—Both Ends of the Busk.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: busk. A piece of whalebone or ivory, formerly worn by women, to stiffen the forepart of their stays: hence the toast—Both ends of the busk.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.